The present invention relates to a manhole pit. More particularly this invention concerns a unit for lining such a pit and for forming a connection between sewer pipes and the like.
It is known to form a unit for lining a manhole pit starting with a base part constituted as a flat base plate up from which extends a short collar provided with laterally, that is horizontally, extending connections permanently fixed to the collar and positioned so as to align with the underground pipes the manhole pit is to join. Thereafter a so-called channel formation is formed inside the tube, having the necessary radius of curvature and inclination depending on the sizes and relative heights of the pipes being connected. This channel formation must be custom-made for each manhole pit, as each manhole pit must meet particular requirements which are rarely if ever the same for two separate locations.
Since it is essential to be able to form this custom-made channel formation on the spot the upstanding tubular collar on the base plate cannot be more than approximately 100 cm high. If it is made higher, for instance 200 cm - 500 cm the mason forming the channel formation must climb into the base part in order to do the job. This necessitates doing half of the job at one time, then waiting for this half to harden sufficiently to stand on, and then doing the other half. In addition working in such cramped quarters is extremely difficult and often leads to an inferior job.
After the channel formation has been built inside the upstanding collar on the base plate the height of the lining unit is increased by stacking a succession of spacing rings or further collars on the collar and securing them together. The top of this stack is closed by a cover plate normally provided with a hole adapted to receive a manhole cover or adapted to support further spacer rings on top of which the manhole cover is held at ground level.
The formation of such a pit lining is indeed relatively complex. The various construction steps must take place one after the other, and usually time must be allowed between the steps in order to permit the sealing material, concrete, or mortar to cure and harden. Furthermore making such manhole-pit linings is often impossible in very cold or inclement weather.
A further disadvantage of the known manhole-pit linings is that they are often insufficiently strong, and also very frequently leak. Thus it is a common failing that a manhole-pit lining which extends below the water table allows water to seep in through the walls into the sewer system, thereby overloading the system and unnecessarily taxing the water table. It is also possible occasionally for sewage to leak out of such an arrangement into the surrounding ground, often polluting nearby wells. Such leakage most often occurs due to damage occasioned when the hole lining is backfilled. The earth and rocks pushed in around such linings often damage them, in particular at the joints between the respective parts, so that a lining which is constructed to be watertight is made to leak when installed.
Finally, a difficulty with the known manhole-pit linings is that it is necessary for the contractor to stock a great deal of different types of parts in order to be able to form the various linings needed. Furthermore the construction difficulties from lining to lining give uneven results so that the quality will vary greatly from one location to another. It is also extremely difficult for a contractor to calculate just how much time each such lining will take to construct, so that accurate estimation of construction costs and time is almost impossible. Furthermore it is necessary to dig a relatively large hole in order to form such devices in situ, with the corresponding problems of holding back the earth around the excavation, diverting traffic if necessary, and even driving sheet piling in very wet locations.